1. Field of the Invention
The invention relates to imaging sheets for making large size, full color images by means of a multicolor electrographic process using a one-pass printer followed by transfer of the image to a receptor surface. In particular the invention relates to imaging sheets comprising a dielectric layer which exhibits controlled release properties for image toners deposited on it.
2. Description of Related Art
Full color reproductions by electrophotography were disclosed by C. F. Carlson in his early patents (U.S. Pat. No. 2,297,691) but no detailed mechanisms were described. Another early patent (U.S. Pat. No. 2,752,833) by C. W. Jacob discloses a method based on a single transparent drum coated with a photoconductor around which a web of receptor paper is fed. Electrostatic images are produced on the drum and by induction on the receptor paper, by three colored line scan exposures from inside the drum. Charging stations precede And toner stations follow each of these scan positions. The final tricolor image is assembled directly on the receptor paper. In U.S. Pat. No. 4,033,688 (Agfa-Gevaert) a single photoconductive drum is exposed to three color beams disposed at different points around its circumference, each point being provided with the requisite charging and toning stations and the three color images are transferred in registration to a receptor sheet. Other similar systems are disclosed in U.S. Pat. No. 4,403,848 and U.S. Pat. No. 4,467,334. The use of a sequence of exposure/toning stations immediately following one another as opposed to multiple drum rotations as found in other methods (eg. U.S. Pat. No. 4,728,983) gives higher production rates for the color prints.
The use of electrographic processes, as opposed to the electrophotographic processes described above, is well represented in the art. In these processes the electrostatic latent image is produced directly by "spraying" charge onto an accepting dielectric surface in an imagewise manner. Styli are often used to create these charge patterns and are arranged in linear arrays across the width of the moving dielectric surface. These processes and the required apparatus are disclosed for example in U.S. Pat. No. 4,007,489, U.S. Pat. No. 4,569,584, U.S. Pat. No. 4,731,542 and U.S. Pat. No. 4,808,832. In all of these, the multicolor toner image is assembled and fixed on the accepting surface. None of these references discloses transferring the assembled image to a receptor surface.
The toners disclosed by C. F. Carlson (U.S. Pat. No. 2,297,691) were dry powders. Staughan (U.S. Pat. No. 2,899,335) and Metcalfe & Wright (U.S. Pat. No. 2,907,674) pointed out that dry toners had many limitations as far as image quality is concerned, especially when used for superimposed color images. They recommended the use of liquid toners for this purpose. These toners comprised a carrier liquid which was of high resistivity (&gt;10.sup.9 ohm.cm) and had both colorant particles dispersed in the liquid and preferably an additive intended to enhance the charge carried by the colorant particles. This basic formulation is still used in the current art.
A number of methods have been disclosed in the patent literature intended to effect liquid toner image transfer with high quality. Silicones and polymers containing silicones as mould release layers and leveling compounds are well known as additives to layers to give release properties.
In the electrophotographic field, photoconductive layers topcoated with silicone layers are disclosed in U.S. Pat. No. 3,185,777; U.S. Pat. No. 3,476,659; U.S. Pat. No. 3,607,258; U.S. Pat. No. 3,652,319; U.S. Pat. No. 3,716,360; U.S. Pat. No. 3,839,032; U.S. Pat. No. 3,847,642; U.S. Pat. No. 3,851,964; U.S. Pat. No. 3,939,085; U.S. Pat. No. 4,134,763; U.S. Pat. No. 4,216,283; and Jap. App. 81699/65.
In addition to patents dealing with separate release layers using silicone release agents, U.S. Pat. No. 3,476,659; U.S. Pat. No. 3,851,964; U.S. Pat. No. 3,935,154; and U.S. Pat. No. 4,078,927 all disclose the use of silicones as additives to the photoconductive layer itself to give release properties towards both toners and inks (electrographic printing plates). The first two of these patents disclose the admixture of silicone oils, waxes, or resins to the photoconductive material. U.S. Pat. No. 3,935,154 discloses block copolyesters containing silicone units in their chains which are useful as release and leveling agents and form compatible admixtures to organic and inorganic photoconductive materials. They are of particular interest in planographic printing materials. In U.S. Pat. No. 4,078,927 planographic printing materials are also disclosed which comprise photoconductive materials containing block copolymers of "soft segments" from a siloxane monomer and "hard segments" from non-siloxane monomers such as polystyrenes, polyvinylcarbazoles, polycarbonates, and polysulfones. These non-siloxane "hard segments" are disclosed as photoconductive under ultra-violet illumination and can be made visible light sensitive by addition of activators or spectral sensitizers. U.S. Pat. No. 4,772,526 (Sep. 20, 1988) discloses photoconductive layer assemblies for electrophotographic systems in which the top layer, either the charge transport layer or the charge generation layer, comprises a block copolymer of a fluorinated polyether and a polyester or a polycarbonate. The surface exhibits good toner release properties because of the presence of the fluorinated polyether, and also "is compatible with the desired functions of the charge generation and charge transport materials".
Dielectric layers for electrographic printing materials also require good toner transfer properties in many processes. U.S. Pat. No. 4,656,087 (Apr. 7, 1987) discloses dielectric -layers for electrographic imaging wherein particulate silica treated with short chain polysiloxane materials is added to the dielectric resin(s). Japanese unexamined patent application JP 57-171339 published on Oct. 21, 1982 discloses a dielectric layer comprising a mixture of an organic silicon polymer containing siloxane bonding as the main chain, and another resin such as acrylic, polyester, or epoxy resins, in the ratio range 1:4 to 4:1 by weight. These layers are disclosed as suitable for heat-transfer of toners, and as having "excellent thermoresistance, releasing properties, durability, flexibility, etc.".